Ultra-short wave electron tube



May 20, 1952 F. LUDl ULTRALSHORT WAVE ELECTRON TUBE Filed Nov. 14, 1945 Patented May 20, 1952 ULTRA-SHORT WAVE ELECTRON TUBE Fritz Liidi, Baden,

"Patelhold Patentverwertungs.

Switzerland, assignor to & Elektro- Holding A.-G., Glarus, Switzerland Application November 14, 1945, Serial No. 628,528 In Switzerland November 1'7, 1944 Claims.

In magnetic field tubes or magnetrons, electrons are emitted from a cathode which, under the influence of the anode voltage and the applied magnetic field, travel along paths between the oathode and the anode. These tubes are used to generate and amplify electrical oscillations with very short wave lengths.

Objects of the present invention are to provide magnetic field tubes which operate at higher emciency than the prior tubes, and in' which the dimensional relationships of the tube elements are such that high efli-ciency is obtained with anode potentials and magnetic field strength which are not of excessive magnitudes. Objects are to provide magnetic field tubes in which the electrons travel along paths between a segmental cylindrical anode and a coaxial cylindrical cathode or a functionally equivalent cylindrical surface, and the number of anode segments is so related to the radii of the inner and the outer boundaries of the electron path that the tube operates at or in the region of maximum efficiency.

These and other objects and the advantages of the invention will be apparent from the following specification when taken with the accompanying drawing in which:

Fig. l is a perspective view, with parts in section, of a magnetic field tube embodying the invention;

Fig. 2 is a schematic view showing the relative dimensions of another arrangement of the tube elements as seen in a plane at right angles to the axis of the assembly;

Figs. 3a and 3b are similar schematic views illustrating the region of variation of the radius of the cathode or equivalent surface for anodes of the same radius but made up of a difierent number of segments; and

Fig. 4 is a curve sheet showing for two tubes the variation of efficiency as a function of the cathode radius.

Fig. l of the accompanying drawing shows an example of a magnetron tube in perspective view and with part of the envelope E broken away. In this figure K indicates the cathode, S the anode segments, and H a hollow cavity resonator serving as the oscillatory circuit. The magnetic field is not shown; it has an axial direction.

In such magnetic field tubes the electrons emitted by the cathode K travel along cycloidallike paths between thecathode and the anode around the cathode, under the influence of the electrical and magnetic lines of force. Due to the effect of the high frequency alternating field existing between the segments S, the electrons moving along the cycloidal paths are subjected to a density modulation, because in the beginning these electrons have their velocity modulated by the alternating field and subsequently this modulation changes-into a density modulation. The electron packets thus formed in turn cause an amplification of the electrical charging of the segments determined by the oscillation process, and this leads to an amplification of the high frequency oscillations.

The present invention concerns an electron tube for ultra short electromagnetic oscillations that has at least one cathode, an anode positively biased with respect to the cathode and consisting of at least eight segments, the anode being cylindrically shaped and arranged coaxially with the tube axis, and a magnetic field directed parallel to the tube axis, whereby according to the invention the anode radius Ta and the radius rs of the cathode element arranged concentrically with the anode are so selected that the following condition is fulfilled:

where p is the number of pairs of anode segments.

In Fig. 2 which shows schematically a crosssection perpendicular to the axis of an electron tube according to another embodiment of the invention, S again indicates the anode segments, H the hollow cavity resonator, K the hot cathode and L the guide electrode. The hot cathode K and the electrode L together form a cathode element which is concentric with the anode. This cathode element can also be replaced by a surface emitting cathode or a Wire spiral having several turns of radius Tx, because in each case the electron mechanism is fundamentally the same. Generally the electrodes L and K possess at least approximately the same potential. The

' significance of the values Ta, TR and p in the foris assumed to be 10 and the anode radius Ta=1 in this case the limiting values for the cathode radius are Tk'=0.8 and rk"=0.6. Both these extreme radii are shown in Fig. 3a. The anode with a radius Ta is indicated by a full circle for the sake of simplicity. With a tube having 10 pairs of segments and an anode radius of 10 mm. the range of high efliciency according to the invention lies between the values rr'=8 mm. and rr=6 mm.

Fig. 3b shows the limits for a tube where 12:20 and 111:1. In this case the limiting radii are rr=0.9 and rk":0.8. They are also indicated by the dash-dot circles.

The invention is based on a recognition of the fact that for the usual magnetic fields and anode voltages the efiiciency of the tube as a function of its dimensions is considerably higher over a certain range than for the rest of the field, this range lying within the limits defined by the fore- I going Expression 1.

The physical causes for the existence of this range of high efficiency can be explained as follows. As already mentioned, the electrons emitted by the cathode travel along cycloidal paths in planes perpendicular to the tube axis between the anode and the cathode element and are thus bunched. If the mechanical dimensions comply with Expression 1, then each electron oi an electron packet is subjected to at least approximately the same conditions as regards the high frequency alternating field when in the same positions on its cycloidal paths; that is the cycloids lie approximately the same as regards the same phase positions of the electrons on the cycloidal paths correspond to the same phase positions of the high frequency voltage.

Furthermore theory and experiments have shown that the efiiciency within the aforementioned optimum range (1) possesses pronounced maximum values in dependence on the dimensions of the tube. In order to obtain a particularly favourable tube the dimensions are therefore so selected that the eificiency of the tube has a maximum. This is especially the case when the expression is at least approximately equal to 2 or 3.

Fig. 4 shows the course of the efficiency 1 of two tubes as a function of their cathode radius Tl:- Both tubes possess a pronounced range of high eificiency, whereby one tube has one maximum value and the other tube two maximum values. A single maxima peak, as indicated by curve A, obtains when the several operating conditions (including anode potential and field strength) are such that the cycloidal paths of the electrons have the form of a complete in-phase roll circle for each adjacent, pair of anode segments. Under other operating conditions which develop cycloidal roll paths spanning a plurality of pairs of anode segments, a plurality of maximum eniciency peaks are possible, as shown by curve B, at different cathode radius values for a given anode segment assembly.

It has been practically impossible to determine beforehand all dimensional values for a-tube which is to operate at a selected frequency and with a reasonably high efliciency. The specialist is now, however, in a position to determine the dimensional values and relative values to obtain the best operation by first selecting preferred values for certain factors and then merely varying at least one of the dimensions within the stated range. For example, if the magnetron designer first assumes a resonator cavity H of a certain shape and size, he has selected a certain anode radius Ta and can compute the number p of pairs of segments which are required to provide the capacitance for tuning the cavity resonator to the desired frequency. With Ta and 72 thus determined, he can compute a value for m which will afford high ei'ficiency in operation. It has been found that even quite small alterations, for instance as regards the diameter of the cathode element, are sufiicient to obtain the desired results.

The conditions for such maximum values are fulfilled when not only the electrons in the same positions on the cycloids are in phase with the alternating field but also when the cycloidal paths of the electrons after one or more complete circulations inside the tube again coincide with the original cycloidal paths.

Another favourable constructional form of tube is obtained when the distance between the centres of adjacent segments in the peripheral direction, the so-called segment pitch (a/Z in Fig. 2) is less than two millimetres. In this way excessive anode voltages are avoided, this being explained as follows.

According to theoretical considerations the efficiency is determined by the magnitude of the magnetic field, that is to say a higher emciency requires a large magnetic field and vice versa. On the other hand the efficiency 1 as regards a polarized anode segments and furthermore the 3 P and y only depends on the ratio 1. /11, whereby y: (Ta7k) and a is twice the segment pitch. The significance of these values can also be seen from Fig. 2. If the efficiency n is now determined by a certain magnetic field, a tube with small segment pitch, that is with a small a, has on account of this dependency also a small anode-cathode distance 11 and thus also a small anode voltage; from the connection between the anode voltage V and the value 1! where H is the strength of the magnetic field and It a constant, it is then clear that when H is constant a reduction in y will also lead to a decrease in V. Conditions are thus such that a segment pitch which is greater than two millimetres results in anode voltages which are unfavourably high in practice.

The relationship (1) according to the invention means that the number of segments cannot be made optional but must have a lower limit. The number of segments must be at least 8. The expression has a maximum value when var- 0 and in this case is equal to so that the minimum number of segment pairs p is determined by the expression Therefore 7111111936 and the number of segments must be at least 8. The number of segments can naturally vary very considerably. Tubes can be made with more than 100 segments.

Tubes constructed according to the invention possess favourable qualities also when a hollow cavity resonator is used as the oscillation circuit for the high frequency currents. Such a constructional form is shown in Fig. 2. In this case the cavity resonator H together with the anode segments S forms a ring, the anode segments being connected to the resonator in such a manner that neighbouring segments are alternately connected to different side walls of the resonator.

In this way the segments form the main capacitive part of the cavity resonator. The wave length of the generated oscillations is determined by the cross-section of the annular-shaped resonator H and the mutual capacitance of the anode segments per unit length along the circumference. The result of this is that the tubes never attain excessive dimensions, even with very short Wave lengths. The assembly of the tubes, also of those designed for ultra short waves, is not unfavourably small. The length of the anode segments does not become excessive even with the longer wave lengths, so that no difliculties are encountered as regards the production of a homogeneous magnetic field. Furthermore the electric losses are low due to the use of a hollow cavity resonator.

The tubes are very suitable for large powers, especially in connection with large surface cathodes. In certain cases it is advisable to replace the large surface cathode by a conductor electrode which is at least approximately a constant distance from the anode and which for at least its greatest part is not penetrated by the electrons, this electrode being combined with at least one hot cathode. Fig. 2 shows such a conductor electrode L together with a hot cathode K. The electron mechanism in such tubes is practically the same as in tubes with a large surface cathode, apart from the differences resulting from the combination conductor electrode with at least one cathode where the electrons are only emitted at discrete points of a cylindrical surface instead of uniformly over the entire surface. These differences are, however, only of a secondary kind.

With certain tubes it is not maximum efficiency but for instance a prescribed value for the magnetic field, which is the chief consideration, so that with such tubes it is an advantage not to select the working conditions in the field of maximum efiiciency but in some other field within the range according to the invention.

A tube with seven pairs of segments, a magnetic field strength of about 600 gauss and an anode voltage of about 1200 volts possesses particularly favourable properties.

I claim:

1. An electron discharge device comprising an annular hollow anode structure constituting a cavity resonator with annular end walls and bounded in a radial direction by two cylindrical walls, the inner cylindrical wall being constituted by a plurality of not less than 4 pairs 12 of interleaved anode segments with alternate segments mounted on opposite end walls of said cavity resonator, the mutual capacitance of the interleaved anode segments and the inductance provided by said end and outer cylindrical walls tuning said cavity resonator to a natural resonant frequency which determines the frequency of operation of said discharge device, and cathode means coaxial with said inner cylindrical wall and defining the inner cylindrical boundary of an unobstructed electron path space of annular cross-section, the outer cylindrical boundary being defined by said anode segments; the anode segment radius Ta and the radius Tk of said inner cylindrical boundary being related to each other and to the number p of pairs of anode segments to satisfy the condition that:

1) T,, 7 k 47 1.8 a( 2 1 o 3.4

2. Electron tube as 11 1 01341111 1, wherein the anode segment pitch is less than 2 mm.

3. Electron tube as in claim 1, wherein the expression 2 r +r 4m o 2 a+ 'h) I is at least approximately a whole number.

4. Electron tube as recited in claim 1, wherein said cathode means forming the inner cylindrical boundary of the electron path space includes a cylindrical conductor electrode having a circum ferential gap and a cathode positioned in said p- 5. Electron tube as recited in claim 1, wherein said cathode means comprise a multiple turn helix of radius Tk- FRITZ LUDI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

